1. Field of the Invention
The present invention relates to connectors for coupling electronic devices. In particular, it relates to connectors having filters for suppressing noise generated from areas both internal and external to the electronic devices being connected.
2. Description of the Related Art
Digital apparatus utilizing semiconductor elements such as integrated circuits have noise-related problems which cause the apparatus to malfunction. The noises often travel through power lines or signal lines external to the apparatus or through antennas, using the aerial propagation path. In some cases, circuit elements within the digital apparatus are destroyed by such noises.
Conventional countermeasures generally taken for solving these noise-related problems include (1) making up a low-pass LC filter circuit by combining capacitors and inductors on printed circuit boards at every signal path within the devices or (2) mounting a low-pass filter which has been formed by combining the elements.
Publicly known capacitors and LC filters, however, have a large residual inductance with respect to circuit ground. Sometimes, as a consequence, it is impossible to satisfactorily eliminate high frequency noises due to the increased residual inductance caused by the wiring on the printed circuit board. To eliminate the increased inductance, therefore, ground terminals must be connected to a plurality of signal paths. Adding the ground terminals, however involves complex wiring designs of the printed circuit boards. Adding elements and wiring to the circuit boards, however, results in increased surface area and increased cost.
For radiated noises, the connector which couples the electronic devices acts as a bridge, allowing the noise to by-pass the noise filter mounted on the printed circuit board. In an effort to eliminate this problem, a connector has been used having an LC filter incorporated therein. Such connector is a shield structured connector which has a filter which uses built-in feed-through capacitors. This design provides for grounding the connector directly to the casing of the device, which is a stable grounded body. This arrangement, therefore, reduces the residual inductance generated at a ground-side of the filter compared with the case where the filter was mounted on the printed circuit board. A satisfactory noise reduction effect can be obtained by shielding the device electromagnetically.
As shown in FIG. 10, in the conventional connector with built-in filter having a feed-through capacitor incorporated therein, a connector pin 3 is fixed in a partitioning plate 2 of a dielectric housing 1, and penetrates therethrough. A conductive shield case 4 having a window 5 is fixed to the housing 1. A feed-through capacitor 6 is inserted onto the connector pin 3 and ring-shaped solders 7 and 8 are applied. The capacitor 6 is soldered to a window edge 5a of the shield case 4 and to the connector pin 3, respectively. A protruding end of the connector pin 3 is securely inserted into a ferrite core 9 for improving filter characteristics.
The conventional connector with built-in filter using a built-in chip capacitor (not shown) is constructed having a plurality of holes into which the connector pins are inserted. Each edge of the holes is formed having a conductor pattern and a common ground-side pattern. A capacitor is connected between the conductor patterns on the printed circuit board, and thereafter the conductor patterns are connected respectively to the connector pin and the shield case.
Such connectors with built-in filters having feed-through capacitors incorporated therein have the advantages described above but also have a number of problems. First, when the feed-through capacitor 6 is soldered to the window edge 5a of the shield case 4 and the connector pin 3, soldering flux seeps into the space between the partitioning plate 2 of the housing 1 and the soldering portion of the shield case 4. The residual flux may degrade the insulating characteristics of the feed-through capacitor 6, after a period of time.
Second, a difference between the thermal expansion coefficients of the shield case 4 and that of the housing 1 may result in stressing and cracking of the feed-through capacitor 6 depending upon ambient temperature variations
Third, generally in the conventional example, to improve the filter characteristics, the connector pins are inserted into a plurality of ferrite beads or a ferrite core in which a plurality of through holes are formed. Since the ferrite beads and the ferrite core are provided on the outside of the shield case after the capacitors are soldered, it is difficult to miniaturize such connector. Furthermore, an additional process is required for positioning the ferrite core which results in increased costs.
Fourth, in the connector with built-in filter, it is desirable to make the spacing between the connector pins small to reduce the size of the connector. This is difficult, however, due to the limitations in (1) the mechanical strength of the feed-through capacitor and (2) limitations encountered in manufacturing.
Fifth, a connector with built-in filter which is capable of eliminating noise at a low-frequency band requires a capacitor having large capacitance. Small, mass-produced feed-through capacitors generally available in the market do not provide as large a capacitance as a unit capacitor. Although a feed-through capacitor of laminate type having a large capacitance is available, it has a substantially higher cost.
In a connector with built-in filter using a built-in chip capacitor, the increase in the number of the components used makes its construction complex. Moreover, since a printed circuit board is used, the residual inductance generated at the ground-side of the capacitor increases, and sometimes the noise at a high-frequency band cannot be eliminated. In order to form a complete electromagnetic shield, a dual-side mounting printed circuit board is required. The dual-side board has ground patterns formed on the entire bottom surface. This, however, results in increased cost.
Furthermore, in order to improve filter characteristics, a ferrite core with multiple holes or ferrite beads must be added after the connector is mounted. This raises its cost due to the increased number of process steps.